1. Field of the Invention
The present invention relates to a cathode ray tube, and more particularly, to a cathode ray tube improving an image quality by maximizing velocity modulation (VM) effects. More particularly, the present invention relates to a slim-type cathode ray tube in which electric fields, i.e. dimensions, are decreased in an anteroposterior direction, and comprises a mounting structure where a VM coil portion, a shape and an icon is improved so as to obtain optimum VM effects.
2. Description of the Related Art
A cathode ray tube is an image forming device employed in a TV or a monitor. The image is formed by a light generated when an electron beam emitted from an electron gun hits a fluorescent surface of a panel of the cathode ray tube. Because the cathode ray tube generally has good color quality, little to no afterimage with respect to a moving image, a high brightness and a wide view angle, the cathode ray tube is widely used in a device displaying a moving image such as a TV. However, a general cathode ray tube usually includes an electron gun with a long electric field and a deflection yoke and requires a high vacuum inner space. Thus, the cathode ray tube requires a significant amount of space due to the long length in an anteroposterior direction, that is in a direction from the front of the cathode ray tube to the rear of the same.
Recently, a slim-type cathode ray tube has been suggested by improving the deflection yoke allowing the length of the cathode ray tube to be shortened in the anteroposterior direction. Significant improvements have been made to shorten the length of the cathode ray tube by shortening a length of other goods such as an electron gun in the anteroposterior direction.
FIG. 1 is a partial cross-sectional view of a conventional slim-type cathode ray tube. The configurations and operations of the conventional slim-type cathode ray tube now will be described with reference to FIG. 1.
First, in general, a cathode ray tube 1 is provided with a shape having a front glass called a panel 2 connected with a funnel 3, and the interior thereof is kept airtight. A fluorescent surface 16 is provided at the rear surface of the panel 2 such that when an electron beam 9 emitted from an electron gun 11 and deflected by a deflection yoke 10 hits a specific position of the fluorescent surface 16, a specific light is emitted to form an image.
Because the interior of the panel 2 and the funnel 3 is in a high vacuum state, constriction by implosion may occur due to an external impact. In order to prevent this, an external contact portion of the panel 2 and the funnel 3 may be made impact-resistant by a reinforcing band 15. A shadow mask 8 performing color classification functions is placed behind the fluorescent surface 16. A frame 6 supporting the shadow mask 8, a spring 5 and a stud pin 4 are also provided in the cathode ray tube 1. Moreover, an inner shield 7 for shielding the effects of geomagnetism during an operation of the cathode ray tube is formed in the frame 6.
The deflection yoke 10 for deflecting the electron beam 9 vertically/laterally is placed outside of a neck portion of the funnel 3 and a color purity magnet 12 CPM for improving a color purity is placed at behind the deflection yoke 10. Here, a rear portion of the funnel 3 is sealed using a material such as a glass and the electron gun 11 is placed within the sealed space. The deflection yoke 10 and the CPM 12 are placed at a rear portion of the sealed funnel 3.
The deflection yoke 10 deflects the electron beam 9 vertically/horizontally toward a specific position on the fluorescent surface 16. The CPM 12 includes a magnet 14 for correcting a convergence of the electron beam and a Velocity Modulation VM coil 13 for improving the image quality.
More particularly, a second differential signal of the image signal is input through a predetermined different circuit in the VM coil 13. The signal input to the VM coil 13 generates electric fields at a circumference portion of the VM coil 13, and the electric fields overlap with a horizontal magnetic field of the deflection yoke 10 to change a horizontal scanning velocity of an image line. The brightness on the screen changes in accordance with the changed velocity. If the deflection velocity is increased, the brightness at the portion is decreased and if the deflection velocity is decreased, the brightness at the portion is increased. Accordingly, since a difference of the brightness at an interface of dark and bright portions on the screen becomes larger than when the VM coil 13 is not included, the interfacial portion becomes clearer resulting in an improved clarity with respect to an outlined portion of the screen. This is called a VM effect.
To obtain the maximum VM effects, it is preferable that the VM coil is positioned at a focus electrode of the electron gun 11, i.e., outside of a G4 electrode of the electron gun. However, if the cathode ray tube is shortened in the anteroposterior direction such as with the slim-type cathode ray tube 1, a position where the CPM 12 can be mounted is limited by the deflection yoke 10, and thus, a position where the VM coil 13 can be placed is also limited. Therefore, the VM desirable effects cannot be obtained. In other words, the slim-type cathode ray tube has a shape so that the length of a neck portion of the electron gun 11 and the funnel 3 is shortened, and the CPM 12 is not placed at the focus electrode position of the electron gun 11 and instead, applies electric fields to other electrodes of the electron gun 11. For example, the VM coil 13 is positioned outside close to G1, G2 and G3 electrodes. In this case, the VM effects are reduced or non-existent.
In order to mitigate the above problems, a current applied to the VM coil may be increased. However, this increases power consumption which is undesirable. It is also possible to shorten the electric field of the deflection yoke by installing the VM coil more towards the front of the cathode ray tube. But in this case, a moving characteristic of the image and a sensitivity characteristic are decreased undesirably.
Another method to form an opening of a main electrode so as to increase the VM effects is suggested in Korean Patent Laid-open Publication No. 10-2003-0005605. However, in this case, the electric effects of an electrode is degraded, an integration of the electron beam is reduced even though the VM effects can be improved. Also, this presents a problem in the manufacturing process.